In the field of silver halide photography, photographic processes for obtaining a positive photographic image without reversal processing for forming positive image is called a direct positive photographic process, and photographic light-sensitive materials for such process are called direct positive photographic light-sensitive materials. Direct positive photographic processes include various types: one type is a process of exposing previously surface-fogged silver halide grains in the presence of a desensitizer and conducting development processing; another type is a process of exposing a silver halide emulsion containing light-sensitive nuclei mainly within silver halide grains and conducting surface development in the presence of a nucleating agent or after overall uniform exposure. The present invention mainly relates to the latter type process.
Silver halide emulsions containing light-sensitive nuclei within silver halide grains, and therefore forming latent image mainly within the grains, are called "internal latent image-forming type" emulsions, and are essentially different from ordinary silver halide emulsions forming latent image mainly on the surface of grains.
In the internal latent image-forming silver halide emulsions, exposed grains do not substantially undergo surface development, since the latent image formed within the grains takes electrons of the nucleating agent into the interior portion of the grains. On the other hand, unexposed grains form a latent image (fogging nuclei) on their surface due to donation of electrons from the nucleating agent, becoming surface-developable. Thus, a direct positive imge is formed by surface nucleation development after imagewise exposure.
The above-described process for obtaining a direct positive image according to surface nucleation development must provide acceptable fundamental photographic properties such as with respect to photographic sensitivity, increased maximum density (D.sub.max), and decreased minimum density (D.sub.min) for the process to be put into use in practical photographic applications, and many patents and applications as described hereinafter have been published and laid open, respectively, describing various embodiments and improvements of photographic factors such as emulsion composition, sensitizing agent, nucleating agent, etc. However, formation of re-reversed negative image, which is a defect with conventional direct positive emulsions, has been an unsolved problem, even though it is a serious problem causing deterioration of image quality. For example, Japanese Patent Application (OPI) No. 66727/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a method for depressing formation of re-reversed negative image, which method is not adequate for solving the above problem. This formation of re-reversed negative image, characteristic of internal latent image-forming emulsions, is particularly serious when light-sensitive materials are exposed with high illuminance, particularly when exposed to sunlight or reflection light from a stroboscope, and a negative image is printed out as blackish brown spots on the light-sensitive film. Thus, effective techniques which depress formation of the re-reversed negative image upon high illuminance exposure, would be highly desirable.